1. Field of the Invention
The present invention relates to control of oil or gas well production in the latter stages of well life and, more particularly, to a differential pressure controller and method for controlling the action of a plunger lift system or oil lift systems, generally known as artificial lift systems.
2. Description of Related Art
Artificial lift systems use a plunger lift in combination with a motor valve to take oil or gas in a tubing of a well, and put it in a sales line. When the motor valve is closed, a differential pressure is created across the valve. This pressure is generated as a combination of the rate at which product (gas) is removed from the downstream (sales or line pressure) line and the rate at which pressure builds up on the upstream (tubing pressure) side of the valve. The line pressure is dependent on several factors including the number and pressure of adjoining gas wells and the type and efficiency of the sales line gas compressor. The tubing pressure is dependent on well bore geometry, well depth, rate of fluid influx, the rate of bottom hole pressure recovery and other factors. A person skilled in the art of artificial lift systems will understand the normal cycling of a plunger in a plunger lift system. In this context, the desired recovery time of a well is the same as the plunger fall time, which is a fixed set point chosen by the user.
The present state of the art for electromechanical control systems in the oil and gas recovery industry can be seen in U.S. Pat. No. 5,427,504 (plunger only), U.S. Pat. Nos. 4,921,048, 4,685,522, 4,664,602, 4,633,954 and 4,526,228. The disclosures of these patents are incorporated into this specification by this reference. These systems suffer from open loop problems that manifest themselves as an inability to compensate for the effects of changes associated with 1) varying well production rates, 2) wear of the lift system components, 3) fluid production, and 4) sales line pressure fluctuations. What is needed is a system that resolves these problems by a single electromechanical control device, when an artificial lift system, such as a plunger lift system, is in use.